How to render a equirectangular non distorted panorama in Blender cycles?
I've tried a couple more times and here are my conclusions:


Equirectangular panorama renders always 360x180 degrees image, so to ensure that our render is not "squished", render aspect ratio must always be 2x1 (width, height)
Right now there's no way to make it 360x20 degrees (Blender 2.71)
If you are desperate for 360x20 (like me), you'll have to use Gimp or other tool to cut 1/9 of your rendered image.
It is very inefficient, so be prepared (1080px high panorama requires rendering of 19440x9720px image, which you'll have to cut to 19440x1080px)


If someone has a more efficient way of dealing with this problem (or I've made a mistake at some point), I'd be happy to hear about it.